A series of HTLV-I cell lines were prepared by in vitro transformation of rabbit peripheral blood mononuclear cells (PBMC) using HTLV-I cell lines of human or rabbit origin as the virus source. Animals injected with certain infected lines developed high temperatures within four days of injection and died by day 8. The majority of rabbit HTLV-I cell lines, including several derived in the same fashion as the lethal line, cause no overt disease. A detailed study of this phenomenon is underway; results here can shed light on why HTLV-I infection in humans causes serious disease in only a small percentage of infected individuals whereas most infected persons remain asymptomatic. In vitro studies show that the lethal cell line causes apoptosis of T lymphocytes whereas other lines activate PBMC in culture. Viral and/or cell components involved in the induction of apoptosis are being studied. Another avenue of investigation involves derivation of HTLV-I molecular clones to use for a study of specific viral genes. One clone was derived from rabbit HTLV- I T-cell line, RH/K30 which contained two copies of integrated provirus; one of these contained an inframe stop codon in the env gene. A genomic fragment containing the other HTLV-I provirus was cloned into lambda phage and sequence determination revealed an intact HTLV-I genome. Several human and rabbit T cell and fibroblast lines transfected with the RH/K30 clone expressed HTLV-I p24 gag protein at levels comparable to the donor RH/K30 line. A transfected rabbit cell line, RL-5/K30, produced type C retrovirus particles with a density of 1.17g/ml; these contained HTLV-I RNA and had reverse transcriptase activity. In addition to HTLV-I expression, RL-5/K30 cells differed from the parent RL-5 line in that RL- 5/K30 showed increased expression of CD25 (IL-2ra) levels and expression of surface class II MHC molecules. RL-5/K30 was able to transmit HTLV-I infection as measured by both in vitro and in vivo assays. Rabbit PBMC and human cord blood mononuclear cells cocultured with irradiated RL- 5/K30 cells became infected with HTLV-I. Rabbits injected with RL-5/K30 cells produced an antibody response to HTLV-I proteins, HTLV-I provirus was detected in PBMC beginning 3 months after injection and blood cultures were positive for HTLV-I p24. Availability of such clones will facilitate functional studies of HTLV-I genes and gene products.